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Selection and Troubleshooting of Flotation Equipment for Produced Water Handling
Dr. John Walsh [email protected]
4 PM CST
29-November 2012
Outline: Broad overview of flotation equipment Mechanism of oil drop capture by gas bubbles Equipment design considerations Features of particular brands and models of flotation equipment Modeling performance for troubleshooting and design Representative performance data discussed with respect to the mechanisms and design considerations.
The term “Gas Flotation” is sometimes used as if it is one technology.
It is not.
There is a wide range of flotation technologies available today.
Depending on the application, there is likely at least one or more models of flotation technology that is most suitable.
95 % separation efficiency & large and heavy
50 % separation efficiency & compact 40 % separation efficiency & both compact and inexpensive.
The goal of this paper is to provide some guidance in selecting the most
appropriate flotation technology for a given application.
Wemco Depurator –
Mechanical Induced Flotation
Petreco –
Hydraulic Induced Flotation
Internal (submerged) Eductor
Natco ISF –
Mechanical Induced Flotation
Pressure Vessel Design
Petreco Unicel –
2 – Stage Vertical
Bottom Feed Co-Current
Hydraulic Induced Flotation
External Eductor
The following table gives the range of variables for commercially available flotation equipment on the market
Parameter Minimum Maximum
Gas/Water Ratio 0.12 8.5
Residence time (minutes) 0.5 4
Height/Width ratio (m high/m diameter) 1 4
Bubble size (microns) 30 1,200
Reject percentage 0.5 7
No particular machine (model of flotation) has all minimum or maximum values. This slide illustrates the differences that can be found in the marketplace from one flotation technology to another.
In the E&P industry, there are several factors that drive the selection of a particular flotation technology. These
include: total cost (including weight, space and chemical)
schedule operability
after-sales service separation efficiency
Among these factors, separation efficiency is often the
most difficult to predict.
The fundamentals of flotation will be discussed. Theory will be combined with field data to explain why some flotation designs provide greater separation efficiency
than others.
8
Flotation within an oil/gas/water Separation System:
production separators vessels or skim tanks
hydrocyclone flotation unit
horizontal multi-stage mechanically induced flotation
vertical single-stage hydraulically induced gas flotation
Integration of Equipment into a Water Treating System:
Flotation is a secondary process. It is always located downstream of primary separators. If hydrocyclones are present in the system, flotation is always located downstream of the hydrocyclones. The reason for this is two-fold: 1) Flotation is typically capable of separating smaller drops than
primary separation and hydrocyclones.
2) The reject flow rate and oil concentration must be accounted for in the overall process flow. This means that the separators must be large enough to handle the flotation reject flow rate.
LP Wells
Wet Oil
Slop Tank
Dry
Oil
Tank
Overboard
Flotation
Wet Oil
Tank
Oil Export
Gas
deGas
BOT
Gas
FWKO
Gas
HP Separator
IP Separator
IP Wells
Gas
Preferred Process Line-up: Ultimately, the reject must be recycled in order to capture the oil contained in the oily water recycle. Here, flotation reject is routed to a Slop Tank for chemical treatment and settling time. This reduces recycled chemical, and provides additional oil/water separation before recycle into the rest of the system.
600 mg/L OiW 40 micron Dv50
20 mg/L OiW 6 micron
Integration of Flotation Equipment into a Separation System:
200 mg/L
12 m
Oil
Oil
Oily water
reject
effluent
HP Wells
To begin the discussion of different flotation machines, we need to have a common understanding of how flotation works.
Bubbles rise and crash into oil drops.
Some of the oil drops are captured by the bubbles.
The bubbles continue to rise to the water surface where they join other
bubbles to make an oily foam.
At the top of the oily water, the foam continuously collapses allowing pools of oil to form.
The oil is swept off, scraped off, weired off, pushed off or allowed to fall
off the surface of the water into a separate trough.
Gas Flotation - Fundamentals
Gas bubbles rise rapidly and collide with the oil drops The collision frequency depends on the concentration of oil drops, the concentration of gas bubbles, and on the projected areas of the oil drops and the gas bubbles.
oil drops
gas bubbles
In addition to collision frequency, capture efficiency is important.
When an oil drop and a bubble collide, the oil drop doesn’t necessarily get captured by the
bubble.
The oil drop can slide off the surface of the bubble, or be carried around the bubble by the
hydrodynamics of the flow.
Whether or not an oil drop gets captured depends on the complex fluid dynamics around the rising bubble, and on the surface chemistry
between the bubble and drop.
Gas Flotation - Fundamentals
Question: what do you think is the fraction of collisions that result in capture of the oil drop?
Answer:
When large bubbles are used (700 micron) capture efficiency is roughly
1 in 10,000.
When small bubbles are used (30 micron) capture efficiency is roughly 1 in 100.
Q: why are these values so low?
A: because gas bubbles rise fast (large density difference with water) and the small oil drops tend to follow the currents which carry them
around the bubbles. The oil drops follow the “streamlines” around the gas bubbles. The larger the bubble, the faster it rises and the fewer oil
drops it will collect.
Ref.: Ramirez & Davis, Separation Science and Technology, v. 36, p. 1-15 (2001)
Physically, gas bubbles and oil drops rise at different rates according to Stokes law: V12
The probability of collision is equal the cross sectional area of bubble and drop: p(do+dg)
2, and the concentration of oil drops (no) and gas bubbles (ng) The capture efficiency accounts for the fraction of collisions that result in capture of an oil drop: E12
where:
n1 number density of gas bubbles (number of bubbles per unit volume)
n2 number density of oil drops (number of drops per unit volume)
a1 radius of gas bubbles
a2 radius of oil drops
V12 relative velocity of bubbles and drops (Stokes Law)
E12 bubble-droplet capture efficiency
Calculation of oil capture:
gas
in
gas & oil
out
1212
2)()ln(
EVddndt
ndbob
o p
)2/3)(/()6//()4/)(/( 32
bwgbbwg dQQddQQ pp
Collision, Attachment, and Capture of
an oil drop:
The size of the oil drop shown here is roughly 10 times larger than realistic. This sequence will be discussed later in relation to the chemistry of the oil/water/gas interaction. For now, it is noted that there are several steps in order for a successful capture of an oil drop.
Collision frequency as a function of the amount of gas that is used and the bubble size:
Collision frequencies are very large numbers compared to capture efficiencies.
So all machines overcome poor capture efficiency to greater and lesser extent by adding chemical and by having a lot of bubbles which dramatically increases the collision frequency.
The greater the amount of gas, the greater the number of bubbles, the greater the number of
bubbles, the greater the collision frequency.
1.0E-03
1.0E-02
1.0E-01
1.0E+00
1.0E+01
1.0E+02
1.0E+03
1.0E+04
1.0E+05
1.0E+06
0 200 400 600 800 1000
Co
llis
ion
Fre
qu
en
cy(c
oll
isio
ns
to
tal)
bubble diameter (microns)
high GWR low GWR
Gas Flotation - Fundamentals
The model allows a theoretical calculation of oil removal efficiency as a function of typical gas flotation variables such as:
Gas bubble diameter Gas/water ratio (sfc gas/ft3 water) Oil drop diameter
Shown in the figure are efficiencies for two types of flotation machines:
Small bubbles / low gas rate Large bubbles / high gas rate Oil drop size: 20 microns; 1 minute residence time.
0
20
40
60
80
100
1 10 100 1,000
sep
arat
ion
eff
icie
ncy
(%)
gas bubble size (micron)
10 % gas
20 % gas
600 % gas
850 % gas
Some equipment provides large bubbles and lots of gas, and other equipment provides small bubbles but not much gas.
Unfortunately, there is no machine that gives small bubbles and a lot of gas.
The reason for this, is partly related to mechanical configuration and to the cost of generating gas bubbles. Generating small bubbles is expensive.
0
20
40
60
80
100
1 10 100 1,000
sep
arat
ion
eff
icie
ncy
(%)
gas bubble size (micron)
10 % gas
20 % gas
600 % gas
850 % gas
The large bubble, high gas volume machines, achieve
high efficiency over a comfortable range of gas rates and bubble sizes.
The small bubble machines deliver much less gas and so bubble size and
gas rate become very important – note the gap in performance
between 10 % gas and 20 % gas. These machines are not so forgiving.
0.01
0.1
1
10
100
10 100 1,000
ga
s /
wa
ter
rati
o(G
WR
, d
ime
ns
ion
les
s)
gas bubble diameter (micron)
Flotation machines
high collision freq
medium coll freq
low collision freq
Design Choice –
amount of gas and bubble size:
Summary of Flotation Mechanism:
Collision Frequency: Gas to Water Ratio Collision frequency more gas = more bubbles. Bubble size Collision frequency small bubbles = more bubbles. Capture Efficiency: Bubble size Capture efficiency small bubbles = slower = more time for coalescence and oil spreading on the gas bubble.
Good oil/water separation = small bubbles, high gas to water ratio. However these are two items are the main cost drivers.
This concludes the sections on Broad overview of flotation equipment
Mechanisms of oil drop capture by gas bubbles
The next section is about chemical treating
After that: Design of Equipment
Models available Performance Data
Modeling
Are there any questions?
23
initial adsorption
polymer colloidal particle destabilized particles floc
secondary adsorption
restabilized particle
initial adsorption (excess polymer dosage)
excess polymers stable particle
rupture of floc
high shear
secondary adsorption restabilized drops
Various flocculant reactions:
+
GOOD BAD
Unicel Flotation Unit Chemical Trials
Oil
con
cen
trat
ion
in
to f
lota
tio
n c
ell
(pp
mv)
Dia
met
er o
f o
il fl
oc
into
flo
tati
on
cel
l (m
icro
n)
Oil
con
cen
trat
ion
o
ut
of
flo
tati
on
cel
l (p
pm
v)
Oil drop size upstream of chemical addition: Dv50 = 25 micron
A: chem 1 @ 16 B: chem 2 @ 12.8 C: chem 2 @ 9.6 D: chem 2 @ 8 E: chem 1 @ 16 F: chem 3 @ 6.5 G: chem 3 @ 9.6 H: chem 1 @ 16 (injection conc in ppmv)
0
10
20
30
40
50
60
70
80
90
100
0 20 40 60 80 100
sep
arat
ion
eff
icie
ncy
(%
)
floc size (microns)
Feed oil drop size Dv50 = 25 micron Effluent oil drop size Dv50 = 14 micron (compare with Mars Wemco Effluent Dv50 = 6 micron)
The Effect of flocculating Agents on Flotation
Unicel: 3 min res time Without chemical, the separation efficiency is 23 %. With proper chemical selection and dosage, the separation efficiency increases to > 80 %. Note the presence of flocs of oil drops in the range of 64 to 84 micron.
1 3 5 7
2
5
10
20
50
100
res
idu
al tu
rbid
ity [
%]
mixing intensity x time
*
*
*
*
+
+
+
+
o
o
o o
x
x
x x
^
^ ^
^
= = = =
Chemical & Dosage (mg/L) chemical
A B C
24 12.8 6.7
26 13.9 7.3
28 15.0 7.8
30 16.0 8.4
32 17.1 8.9
34 18.2 9.5
Chemical mixing / time / concentration:
Turbidity is a measure of flocculation and settling – the lower, the better. Longer time or higher concentration can overcome poor mixing but with adverse consequences – gunk, accumulation, organic and chemical scaling.
1.00E-05
1.00E-04
1.00E-03
1.00E-02
1.00E-01
1.00E+00
0 500 1000 1500 2000
Cap
ture
Eff
icie
ncy
(ca
ptu
res/
colli
sio
n)
bubble diameter (microns)
without chemical
with chemical
… but with chemical, capture efficiency improves significantly, particularly for large bubbles:
The use of chemical significantly improves capture efficiency. This is why chemical is so important for flotation.
With chemical, capture efficiency is roughly 1/100 for large bubbles.
It is 1/10 for small bubbles.
An important point to notice here: Without chemical, capture efficiency is proportional to 1/d2
With chemical, capture efficiency is proportional to 1/d We will return to this later.
Good bubble distribution,
good froth, poor oil drop
attachment and capture.
Best Practice:
spreadingfor 0 SC
tensionlinterfacia:γ
tcoefficien spreading:SC
)(
ogowwgSC
Problem:
Besides chemical, a bit of mild turbulence improves the capture efficiency.
Turbulence makes the trajectory of the oil drop and gas
bubble more random and hence the oil drops deviate from the hydrodynamic streamlines.
Large bubbles create more turbulence.
Rotating paddles also create turbulence.
Swirl motion suppresses secondary motion and eddies.
Coalescing elements near the top provide surface area for oil
drop coalescence.
Other Factors:
This concludes the section on chemical treating
The next section is about equipment
Are there any questions?
A. horizontal multistage 1. mechanical induced gas
a) underflow weir (e.g. Wemco) b) overflow/underflow weirs (e.g. Cica, Cameron ISF)
2. hydraulic induced gas 1. submerged venturi (e.g. Enviro-Tech Systems) 2. external venturi (Petreco)
3. dissolved gas (mostly used downstream)
B. vertical 1. hydraulic induced gas
a) internal (submerged) venturi (e.g. Versaflo) b) external venturi eductor
i. bottom feed co-current (e.g. Unicel) ii. bottom feed counter-current iii. top feed (e.g. Epcon)
c) multistage (e.g. TS Technologies) 2. dissolved gas
a) external pump i. bottom feed co-current ii. bottom feed counter-current iii. top feed (e.g. Siemens SpinSep)
Types of Flotation Units:
Note that no eductor is shown.
Wemco Flotation Unit
Wemco Depurator was the original IGF design
introduced to the oilfield in the 1960's.
Typical gas blanket pressure is roughly 0.5 to 4 ounces.
Siemens Veirsep Multi-Stage Horizontal DGF Unit Horizontal Multi-Cell Hydraulic Induced Flotation:
Enviro-Tech Systems – Submerged Eductor
Oil removal is a
function of physical
and chemical
characteristics
(kinetic rate
constant), residence
time and number of
cells.
Note: single cell
efficiency is only
about 50 %.
Klimpel (Flotation) Kinetic
Analysis:
Multi-Stage Theory of Flotation Units
k = kinetic rate constant (typically 1.2)
n = number of cells (typically 4)
t = flotation cell residence time (typically 1 minute)
n
in
n
ktC
C
1
1
n
ktE
1
11
Ursa
Mars Auger Eductor cleaning &
increased reject rate
Ref: Movafaghian et al, Petreco, SPE 81135 (2003)
Wemco Horizontal Rotor-Type IGF as a Benchmark:
Wemco Performance: Mars platform Deepwater GoM Note: average oil separation efficiency was 92 % Feed to Wemco was from the discharge of free water knockout.
70
75
80
85
90
95
100
Se
p-0
3
Oc
t-0
3
No
v-0
3
De
c-0
3
Ja
n-0
4
Fe
b-0
4
Ma
r-0
4
Ap
r-0
4
Ma
y-0
4
Ju
n-0
4
Ju
l-0
4
Au
g-0
4
Se
p-0
4
Oc
t-0
4
No
v-0
4
De
c-0
4
Ja
n-0
5
We
mc
o S
ep
ara
tio
n E
ffic
ien
cy
1
10
100
1000
Oc
t-0
3
No
v-0
3
Ja
n-0
4
Ma
r-0
4
Ap
r-0
4
Ju
n-0
4
Au
g-0
4
Se
p-0
4
No
v-0
4
De
c-0
4
Fe
b-0
5
FW
KO
Dis
ch
arg
e &
We
mc
o D
isc
ha
rge
Oil &
Gre
as
e (
pp
mv
)
Induced Gas Flotation Unit – Gannet
10
100
1000
10000
1
52
103
154
205
256
307
358
409
460
511
562
613
664
715
766
817
868
919
970
102
1
107
2
112
3
117
4
122
5
127
6
132
7
137
8
142
9
148
0
153
1
158
2
163
3
0
10
20
30
40
50
60
70
80
90
100
IGFU In
IGFU Out
efficiency (%)
40
OiW
mg
/L
%
removal
The inlet and outlet oil concentrations are shown in the blue and purple dots and the left hand
scale. The target oil concentration in the discharge was 40 mg/L which was achieved most of
the time. Relatively high oil concentrations were fed to the unit. Also, there was significant
variation in oil concentration to the unit. Oil removal efficiencies were generally good but
somewhat variable, likely due to the variation in oil content going in.
0
10
20
30
40
50
60
70
80
90
100
0 50 100 150 200
We
mco
Eff
icie
ncy
(%
)
Wemco Feed Oil Concentration (mg/L)
Wemco efficiency – as the oil content goes down, as a result of better upstream processing, the average drop size goes down as well. This results in less oil, and smaller drops being fed to the Wemco. The net result is a decrease in Wemco separation efficiency.
Wemco Efficiency as a Function of Feed OiW Concentration:
This figure suggests that drop size as small as 3 to 6 micron are effectively removed in a Wemco model flotation unit, with the use of chemical, and 2 % reject.
Ref.: Leech et al., JPT(1980)
Wemco reject flow rate automation – an automated feedback control system was installed in order to reduce the variability in the reject flow rate. Much of the variability was due to sea state. Regardless of the source, the feedback system was very effective in eliminating the variability and allowing the Wemco to operate with a steady reject flow rate.
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
10,000W
em
co R
eje
ct F
low
Rat
e (
BW
PD
) Wemco Reject Rates (BWPD)with and without process control
process control applied
Wemco Reject Flow Control:
MBD – 250
FI
LCV
SP
PID
Wemco level control: a flow meter (FI) sends the flow rate to a PID controller which compares the measured flow rate to a set point. The controller then sends a signal to the level controller for the effluent discharge which adjusts the level control set point.
Reject for upstream
recycle
Effluent for overboard
To upstream recycle
overboard
Feed
Wemco Reject Flow Control:
Dissolved Gas Flotation Configuration is similar to that of dispersed gas flotation. Effluent water is recycled. Gas is dissolved (or injected) into the effluent water and fed into the flotation unit. Unit is designed to handle recycle water volume.
Horizontal Flotation - DGF:
42 Ref: Yang, Galbrun, Frankiewicz, SPE – 90409 (2004)
Fluid Flow / Hydrodynamics:
Illustrative example: since most horizontal flotation equipment is large, it is typically designed for high flow rates. This creates a cross flow. Bubbles that are too small to rise against the cross flow get swept from one chamber to the other and eventually to the discharge without providing flotation. On-shore installations (e.g. refineries) operate at much lower flow rates and therefore much lower bubbles size can be used, such as dissolved air flotation.
oil contaminated
water feed
gas feed
oil outlet
water
discharge
Pressurized Cylindrical Design IGFU
Box-style units tend to
have quite a bit of
“fugitive emissions.”
Pressure vessels
eliminate the loss of
hydrocarbon vapor to
the atmosphere, but
typically do not have
view ports.
Pressure Vessel Designs:
Skim Wing
Skim Door
Flow
Cylindrical WEMCO ® Depurator ® Hydraulic Skimming • No Internal Skimmer Assembly • Skims by Fluid Motion
Equipment Arrangement Cylindrical Wemco ® Depurator ®
Cameron Wemco Depurator – Pressure Vessel Design:
This is the end of the section on Horizontal Multi-Stage Flotation
Next Section: Vertical Flotation
Are there any questions?
Vertical Flotation:
Petreco Unicel (now Cameron) Monocep SpinSep (now Siemens) Cetco CrudeSep (now Cetco-Monarch) Natco VersaFlo (now Cameron) EPCON CFU (now MI Swaco)
Most vertical flotation units are intended to be smaller and to weigh less than a
Wemco, and therefore typically have lower residence time and are only single or
“double” stage.
Care is usually taken to ensure small bubble size. Dissolved gas, which gives
very small bubbles can be effective.
To overcome the limited residence time and the lack of multiple stages, some
units have coalescing elements in addition to flotation, which require that the oil
be evaluated for stickiness.
Design considerations include:
• means of generating and introducing the bubbles
• configuration for feeding and discharging the water and oil.
47
0 20,000 40,000 60,000 80,000 100,000
1
10
100
1000
1
10
100
1000
0 100 200 300 400 500 600 700
capacity (BWPD)
we
igh
t (m
etr
ic t
on
s)
capacity (m3/hour)
Natco VersaFlo
Natco horizontal IGF
Unicel
Epcon CFU
Weight of Horizontal vs Vertical Flotation:
Types of Flotation Units:
In the mechanical design of a flotation unit, there are a few practical
considerations.
First, the oily water must be introduced in a way that does not disrupt the
flotation process.
Second, the oil and clean water must be discharged in a way that does
not allow contamination with the feed and that does not disrupt the
process.
Third, the overall weight and space must be as low as possible, which
achieving a high separation performance.
Practical Considerations in Design:
Unicel –
Hydraulically Induced Gas Flotation
Bottom Feed Co-Current
Vertical Flotation - Unicel:
Unicel Vertical Induced Gas Flotation Unit - Pumpless Unit
Compact Vertical Flotation - Unicel:
Typical designs provide sufficient pressure for the inlet produced water (> 45 psig) to charge the eductor, thus typically no recycle or inlet pump is required.
High volume / low feed pressure
system do require a recycle pump. Water with dispersed gas bubbles enter
at bottom of riser pipe. There is a coalescing element at the
top of the pipe. An oily froth is spilled over the weir
into the skim through, which is emptied periodically by open a discharge valve.
Typical Eductor for Unicel:
Gas Inlet
Eductor Water Inlet
Water with Dispersed Gas Bubbles
An eductor may be followed downstream with a dispersing
element such as a static mixer
Induced gas flow rate is calculated using Bernoulli’s principle
Unicel performance example:
Single cell, external recycle provides flow through a venturi
Design feed flow rate: 16,000 m3/day
Design residence time: 34 seconds at design flow rate
Design gas / water ratio: 0.14 m3 gas / m3 water
0
5
10
15
20
25
30
35
40
45
0 20 40 60 80
sep
arat
ion
eff
icie
ncy
(%
)
residence time (seconds)
after chemical
optimization
53
flocculent
injection
Epcon (CFU)
(now MI Swaco, a division of Schumberger)
10
100
10 100 1,000
se
pa
rati
on
eff
icie
nc
y (
%)
inlet OiW concentraion (ppmv)
Hudson & Tern Alpha
Brage
Heidrun
Draugen
North Cormorant
Troll C
Epcon CFU Performance:
32
12
23
outlet
OiW
for
trend
line
inlet (mg/L) sep eff (%) outlet (mg/L)
80 63 30
40 50 20
55 Ref: Bothamley, SPE-90325 (2004)
North Sea vs Deepwater GoM Liquids Processing Systems:
hotter fluids
all 3-phase w/ hydrocyclones
less shear
cooler fluids
2-phase / 3-phase
more shear
10
100
1,000
10,000
1 10 100 1,000
OiW
co
nc
en
tra
tio
n (
pp
mv
)
drop diameter Dv50 (micron)
56
Field data together with modeling demonstrates why flotation is required in
the deepwater GoM but not in the North Sea – both shear and fluid
temperature are significant factors.
North Sea vs Deepwater GoM Process Performance Diagram:
flotation
hydrocyclone
3f FWKO
hydrocyclone
3f Sep
GoM
NS
Ref: Walsh & Georgie SPE – 159713
Vertical DGF - Spinsep: Siemens SpinSep Dissolved Gas Flotation:
TS Technology – Multistage Vertical Induced Gas Flotation
Compact Vertical Flotation - CrudeSep:
GLR (Exterran) – Dispersed Gas Pump & Skim Tank
Micro Bubble Demonstration Tank
IGF vs GFT (Microbubble):
Theoretical analysis indicates that hydrocarbon removal should improve with increasing bubble
concentration and reduced bubble diameter. This is the principle behind DGF, Epcon, GFT.
IGF (Wemco):
Typical gas/water ratio: 8.5 m3 gas/m3 water
Typical gas bubble size: 100 to 1,000 microns
GFT:
Typical gas/water ratio: 0.12 m3 gas/m3 water
Typical gas bubble size: 20 microns
Comparison of number of bubbles:
However, field performance of a Wemco is difficult to improve upon. This is
due to having windows that allow operators to see what is happening for
troubleshooting and to make adjustments, and a multistage design (Klimpel
eqn). These are important factors.
IGF
GFT
dv
dv3
3
/
/IGF Bubble Size Ratio
100 2
380 100
1000 2000
Depending on the IGF bubble size,
GFT has potentially a much higher
number density of bubbles.
Vertical Gas Flotation Systems Key Drawbacks: • Typically low oil removal efficiency (roughly 50 to 60 % per stage) • Rely on coalescence which can be rendered useless when oily solids are present • This technology is best deployed downstream of other “coarse” separation equipment e.g
desander and /or deoiler hydrocyclones. • Vertical gas flotation units require continuous injection of a suitable water clarifier/
flotation aid polymer chemicals to achieve high water treatment specifications • There is usually no “quiet zone” for chemical reaction
• Feed water streams with high clay and silt fines content can be a problem if they settle inside the gas flotation vessel and oil box as the sediment can reduce residence time and cause maldistribution of gas.
• This type of design does not react well to flow surges. Performance tends to deteriorate rapidly under these conditions.
• For applications where Calcium Naphthenate is present blockages can be expected within the vessel internals.
Compact Vertical Flotation – Benefits & Drawbacks:
Vertical Flotation System Key Benefits: • Small footprint. • Typically high mechanical reliability since there are no internal moving parts. • Usually responds well to upsets and has good turn down capability.
Vertical Flotation – Benefits & Drawbacks:
This is the end of the section on Vertical Flotation
Next Section: Modeling
Are there any questions?
63
Sweep Factor:
Ref: Lee, Frankiewicz, SPE-90201 (2004)
cellgasgas A/FASF
min) cell gas/m (mFactor Sweep :SF 22
)(m cellcontact of area sectional- x:A
min) / gas (m rate flow c volumetrigas :F
gas) of gas/m (m gas of eunit volumper bubbles ofsection - x:A
2
cell
3
gas
32
gas
bb
b
dd
d
2
3
)6/(
)4/(A
3
2
gas p
p
SF: the cross-sectional area of gas that sweeps through a
cross-section of the flotation cell in a minute
64
Sweep Factor:
bd/)A/F(5.1SF cellgas
SF: the cross-sectional area of gas that sweeps through a
cross-section of the flotation cell in a minute
100
1,000
10,000
100,000
1,000,000
10,000,000
10 100 1000
Sw
ee
p F
ac
tor
(bu
bb
le a
rea
/ c
ell
a
rea
m
inu
te)
gas bubble diameter (micron)
GWR = 0.1
0.5
1
5
gas
in
gas & oil
out
cleaner
oily water
out
oily water
in
)/1/(
)statesteadyat(0
QkVCC
kCVCQQCdt
dCV
in
in
)(1/secconstant ratereaction :
)(m cellcontact of volume:
sec)/ (m rate flow:
)m / drops oil (no. oil ofion concentrat:
3
3
3
k
V
Q
C
Quantitative Modeling:
Ideal Continuous Stirred Tank Reactor:
kV/Q)(
kV/Q E
CCCE
QkVC
inoutin
in
1
:ngsubstituti
/)(:definitionby
)/1/(C :CSTR Ideal out
Quantitative Modeling con't –
Separation Efficiency:
1.0
as kV/Q
becomes large
kVk
kV/Q)(
kV/Q E
)k(Q
k E
AS
AS
ASAS
:gives ASh Theory wit Kinetic CSTR comparing
1
Quantitative Modeling con't –
Comparison with Arnold & Stewart:
Arnold & Stewart:
CSTR Kinetic Theory:
Quantitative Modeling con't –
Comparison with Arnold & Stewart:
Arnold & Stewart give: )/)(/(2
wgbpAS QQdHDkk p
efficiency capture & collision :k
(m)diamter bubble:d
(m3/sec) water of rate flow volumetric:
(Sm3/sec) gas of rate flow volumetric:
(m) cellcontact ofheight :
)(m cellcontact ofdiameter :
p
b
w
g
Q
Q
H
D
Note the similarity with the Flux Factor: )/)(/( wgbF QQdHN
)1( x x attachcaptcollp -EEEk
69 Ref: Sarrot et al., Chem. Eng. Sci. (2005)
dc/2
Calculation of Collision Efficiency:
2222 // bcbccoll ddrrE ppdiameterbubble:
diametercollision:
efficiencycollision:
b
c
coll
d
d
E
grazing
trajectory
Ref.: Parkinson, Ralston, Adv Coll Int Sci, v. 168, p. 198 (2011)
Stokes Law – solid
particle or
contaminated bubble
surface, e.g.
surfactant coated
bubble
Clean bubble
surface.
Rise velocity =
3/2 Stokes Law
b
p
colld
dE 3
2
3
b
p
colld
dE
Sutherland Eqn Gaudin Eqn
Ref.: Parkinson, Ralston, Adv Coll Int Sci, v. 168, p. 198 (2011)
b
p
colld
dE 3
Sutherland Eqn (5)
2
3
b
p
colld
dE
Gaudin Eqn (7)
The discussion presented here is useful for judging the design of one
flotation unit versus another.
A good correlation was found between separation efficiency and Flux
Factor times Capture Efficiency.
In the data shown, chemical was used.
Other factors are also important such as oil drop size, presence of
coalescing elements or internal baffles, etc. These are not accounted
for.
We do not wish to show which vendors give which results – call us to
discuss.
... except to say that the horizontal, multi-stage IGF gives the highest
Flux Factor and gives the highest separation efficiency, despite having
large gas bubble diameters. The large volume of gas and the
multistage effect (J. Chen et al.) overcomes the detrimental effects of
large bubbles.
Gas Flotation - Fundamentals
The time that the water spends in the contact cylinder is given by:
tc = contact time (time that oily water and bubbles are in contact (minutes)
Vc = volume of the contact cell (m3)
Qw = volumetric flow rate of water (m3/minute)
WCC QVt /
3/ gCg dAQtimeunitperareaunitperbubblesofnumber
dg = diameter of gas bubble (micron)
Qg = volumetric flow rate of gas (m3/minute)
Ac = cross sectional area of the contact cell (m2)
2sec gdbubblesofareationalcross
Combining these terms gives the Flux Factor: )/)(/( wgcF QQdHN Vc = volume of the cylindrical contact cell
Ac = cross sectional area of the contact cell
Hc = height of the contact cell
The Flux Factor is a dimensionless parameter which characterizes the
mechanical design of a flotation unit.
The Flux Factor gives a measure of the effectiveness of the design
of a flotation machine but it does not account for capture efficiency.
Capture efficiency (E12) can be included in the correlation.
Here we use Parkinson, Ralston model.
(H/d)*(Qg/Qw) x E12
10
100
1 10 100 1000
Sep
arat
ion
Eff
icie
ncy
(%)
Flux Factor x Capture Efficiency
A smooth
correlation exists
between
separation
efficiency and
Flux Factor times
Capture Efficiency
This gives a way
to estimate the
separation
efficiency of a
particular flotation
design
Gas Flotation Technology Summary
Advantages
Well established technology
Can remove suspended solids as well as oil
Good turndown (longer residence time, improved separation)
Relatively low OPEX costs
Highly effective when used with chemical aids
Can cope with a wide range of oil feed concentrations
Disadvantages
Horizontal Units are motion sensitive, vertical are less-so
Units tend to be large and bulky
Maintenance can be high
Some designs use moving parts
Larger solids particles can settle at the bottom of the unit
Eductors, venture can become fouled or plugged
For high water flow rates, may need to install
numerous units in parallel. Smaller models
(<85 kBPD) tend to work better. Should also
consider series arrangement.
Best Practice for Flotation Units:
• Use gas blanket rather than atmospheric air to avoid corrosion, scale
• skimming under varying flow conditions
• Use adjustable skimmers to optimize skimming of oil at surface
• Clean eductors regularly since they can become clogged with scale
• Install anti-motion baffles where required
• Performance is sensitive to chemicals, over treating, contamination, solids
• Do not use sparge tubes
Ref: Jane Pederson, Allan Lawrence, Zara Khatib, “Best Practice in Produced
Water Treatment, SIEP 99-5806
